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September 18, 2014

EWG’s Consumer Guide to Seafood: Mercury Toxicity

Methyl mercury is toxic to the human brain, kidney, liver, heart and nervous system. Research has shown that people who eat a lot of high-mercury fish frequently can experience nervous system damage and can suffer from a variety of ailments, including sleep disturbance, headache, fatigue, difficulty with memory and concentration, poor coordination and neuropathy (Hightower 2003, Silbernagel 2011, Groth 2011). Symptoms of mercury toxicity generally subside slowly once people stop eating fish high in mercury (Hightower 2003).

Mercury is much more dangerous to the developing fetus. Mercury exposure during pregnancy can cause lasting deficits in the development of a child’s brain and nervous system. Studies of children exposed to high levels of mercury in the womb indicate that they score lower than other children on intelligence tests and perform poorly on tests of memory, attention and hyperactivity (Debes 2006, Grandjean 2012, Sagiv 2012). Mercury exposure during infancy and early childhood is also believed to be harmful.

In 2001 the EPA concluded that a pregnant woman could consume 0.1 micrograms of mercury per kilogram of bodyweight daily without ill effects to her fetus and that this amount of mercury would also be safe for children and adults (EPA 2001).

Since then, however, several studies have found measurable damage to infants’ brain development in mothers exposed to lower levels of mercury. Many of these studies found that omega-3s in seafood could offset the deleterious effects of mercury to some degree. A study of 341 Boston-area women found that the 10 percent of women with the highest blood-mercury measurements gave birth to children with poorer brain development, measured at 18 months and three years (Oken 2005, 2008). Those mothers who ate three or more seafood meals weekly gave birth to children with better brain development. The authors concluded that “recommendations for fish consumption during pregnancy should take into account the nutritional benefits of fish as well as the potential harms from mercury exposure" (Oken 2008).

Several scientists and advocates specializing in mercury damage have concluded that the EPA’s safe level is too lax to protect the developing fetus (Bellinger 2014). Some have recommended that the EPA lower its mercury exposure level by 50 to 75 percent (Grandjean 2012, Zero Mercury 2012).

The EPA has launched a multi-year process to revise its assessment of mercury toxicity. EWG believes the agency should lower its safe level to account for the recent scientific studies that have found that children’s intellectual abilities suffer even at trace exposure to mercury. The agency should accelerate its efforts to encourage people who eat fish frequently to seek out varieties lower in mercury.

In the meantime, since government and independent scientists have not reached a consensus on a safe level of mercury exposure, EWG’s Seafood Calculator and Wallet Guide aim to steer people toward seafood with the best safety profiles, something we can do with confidence.

EWG recommends that pregnant women and children consume no more than 75 percent of EPA’s safety level. By doing so, they are likely to build a extra margin of safety from lifelong damage mercury can inflict on the developing brain.